The present invention relates to a circuit for preventing a microcomputer from malfunctioning, in particular to a circuit which functions both to stop the runaway of a microcomputer and to protect the peripheral circuits of the microcomputer from overcurrent.
Circuits for preventing a microcomputer from malfunctioning are known, which detect a runaway condition of the microcomputer by way of the operation of a timer called a "watchdog timer", and then reset the microcomputer. The most commonly used of such circuits includes a counter which is provided outside the microcomputer. When the counter overflows, the microcomputer is reset. Specifically, when the microcomputer is operating normally, the microcomputer supplies a counter-reset signal before the counter overflows; therefore, the counter never overflows. If the microcomputer becomes runaway, the microcomputer does not supply a counter-reset signal. Then, the counter overflows and the microcomputer per se is reset. When the microcomputer is reset, the program counter is reset, and the interrupt operation, input/output port latching, and command register are initialized. As a result, the runaway condition of the microcomputer is stopped. Such a watchdog timer does, however, require a large area on the circuit board.
Various peripheral circuits are connected to the microcomputer. In such peripheral circuits, an over-current can flow due to circuit trouble, possibly damaging the circuit components. To prevent this scenario, various protection circuits have been developed which detect the overcurrent when it is generated, and turn off the power supply to the peripheral circuits.
The watchdog timer for preventing the microcomputer from malfunctioning, and the overcurrent protection circuit for the peripheral circuits of the microcomputer, are arranged on the board as separate circuits. These circuits occupy a large area on the board, resulting in a higher manufacturing cost.